# PowerPoint Presentation - University of Iowa Astrophysics by yurtgc548

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```									             Gamma-Ray Bursts
• Energy problem and beaming *
• Mergers versus collapsars
• GRB host galaxies and locations within galaxy
• Supernova connection
• Fireball model *
• Swift
• Afterglows of short bursts
Energy Problem
• GRB 990123 required a total energy, if
isotropic, of 3.41054 erg = 1.9 M c2.
• GRB energy source is almost certainly
gravitational – need few M collapsed into
region not more than 100 km across.
• Energy density U = T4/c
 T ~ 1012 K kT ~ 100 MeV
This is high enough to produce e+e- pairs.
Fireball
• Consider pure energy confined within a sphere,
describe with E, R, T (Goodman 1986)
• Radius of sphere R = (3E/11T4)1/3
• Optical depth from center to edge
1/ 3
 T                   15    E   kT 
4                                          5/3


 kT       T R  4 10  51
                10 erg   1 MeV 

                                         
• Edge of sphere (photosphere) will expand at a speed
close to c as long as kT > mec2
• If baryons are added, most energy goes into
accelerating baryons to ~ E/Mc2
Fireball
• Optical depth from center to edge for burst which
varies over time scale t with a sepctrum such that a
fraction fp of the photon pairs can pair produce.

2
 E  T 
  10 f p  50
11
 10 erg  10 ms 

               
• Very high optical depth is inconsistent with non-
thermal spectrum at high energies
Relativistic outflow
• In a relativistic outflow, the observed photon energy is
a factor  (= Lorentz factor of bulk motion) higher
than the photon energy in the rest frame. For a
spectrum with an energy index  this reduces the
number of photon pairs above the electron-positron
threshold by –2
• Also the size of the emitting area can be larger by a
factor 2
ct        R
v  c 1    ct  vt  2  t 
2

2        2c 2
• Need  ~ 100 to solve the problem.
Evidence for Jet
Afterglow of GRB 990123 shows a break
Beaming
Because of relativistic motion, radiation is beamed
with an opening angle ~ 1/
Therefore, observer can see only a limited piece of an
expanding shell

Observer

1/ 
At Early time:  1  

 = jet angle

 1

Area visible to an observer = (R/)2
R

At Late time:  1  

 1


Area visible to an observer = (R)2
R
Monochromatic Jet Break
Jet Breaks
• Jet opening angle is related to time at which
break in light curve occurs

• Beaming fraction is determined by jet opening
angle = 1 – cos  2/2
• Energy required is reduced by a factor 2/2
Jet Energy

Frail et al. 2001
Burst Models

•   Collapsing WDs
•   Stars Accreting on AGN
•   White Holes
•   Cosmic Strings
•   Black Hole Accretion Disks
I) Binary Mergers II) Collapsing Stars
Mergers
Binaries must evolve
before merger and binaries
have non-zero speeds due
to kicks in compact object
formation.
Thus, GRBs can occur in
outskirts of or even far
from host galaxy.
Massive Star Collapse

 Beamed Explosion, accompanying supernova-like explosion,
GRBs should be associated with young, massive stars.
Host
Galaxies

Holland 2001

Hosts are similar to star-forming
galaxies at similar redshifts.
High star formation rates.
Location of GRB within Host
Location of
Distribution
GRB within                  Follows
Host                     Stellar
Distribution

The environments of
GRBs show higher gas
densities, higher
metallicities, and higher
dust content than
random locations in
host galaxies.
Suggests that GRBs
occur in star forming
regions.
GRB Locations
• GRB hosts are star-forming galaxies
• GRBs trace the stellar distribution (in distance
from galaxy center)
• GRBs occur in dense environments (probably
star forming regions)

• Suggests collapsar model over merger model
Supernova connection
SN 1998bw was found in the 8’ error circle of GRB
980425 in observations made 2.5 days after the burst.
A slowly decaying X-ray source was subsequently found
in the same galaxy (z = 0.0085) and identified with the
GRB.
However, the GRB was very underluminous and the SN
was very usual with parculiar line emission (no H, no He,
no Si at 615 nm.
Radio emission a few days after GRB indicated
relativistic outflow with energy ~ 31050 erg.
Thought to be oddball GRB and SN.
GRB030329 and SN 2003dh
Clear spectroscopic
signature of a SN, broad
emission lines, found after
decay of afterglow of
GRB030329.
GRBs and SNe.
SN 2003dh versus SN 1998bw
SN Bumps
GRB - Supernova

Only a tiny fraction of SN are observed to be GRBs
GRB 060218 = SN 2006aj
Fireball Model

Initial event accelerates baryons in bulk
Later on, internal shocks re-accelerate particles
produce GRB
Even later, external shocks produce afterglow
GRB 990123

990123 reached 9th magnitude for a few moments!
First optical GRB afterglow detected simultaneously
Internal-External Shock Model
External Shocks
Internal Shocks
ISM
Central Engine

R  1014 cm            R  1016 cm

GRB                       Afterglow
３
Burst (as Jet) Properties

3. Baryonic mass content of the jet ~ 2x10-7   6x10-6 Mo

Baryon mass is ~ 10-5 M
Jet opening angle means that we observe only one of each 1000
GRBs in the Universe, most are pointed away.
The means that GRB rate is about 1% of SN rate.
Swift
BAT – CZT detector with
5200 cm2 area sensitive in
15-150 keV band.
Coded aperture imaging of
1.4 steradian field with 4
arcmin resolution suing
32768 pixels.

After detecting a burst, Swift autonomously repoints bringing the
burst into view of the XRT and UVOT, often within 90 seconds.
XRT – focusing X-ray telescope in 0.5-6 keV band, 2.5 arcsecond
source location accuracy.
UVOT – focusing UV/optical telescope.
Swift Results
• Launched in 2004.
• Detects about 100 bursts/year
• More afterglow detections than all previous satellites combined
• GRB with redshift of z = 6.29
• Average redshift = 2.7 compared to pre-Swift <z> = 1.2
•
• Expect 40 GRB with z > 5 and 4 with z > 8.
Afterglow of short GRB

GRB 050509b associated with elliptical galaxy.
HETE-II GRB 050724 also associated with elliptical.

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